fix: allow storage_billing_model to be explicitly set to None to use project default value (#1665)

* fix: allow `storage_billing_model` to be explicitly set to `None` to use project default value

* add STORAGE_BILLING_MODEL_UNSPECIFIED to docstring

Author: tswast

google/cloud/bigquery/dataset.py

@@ -769,9 +769,10 @@ def storage_billing_model(self):
         """Union[str, None]: StorageBillingModel of the dataset as set by the user
         (defaults to :data:`None`).
 
-        Set the value to one of ``'LOGICAL'`` or ``'PHYSICAL'``. This change
-        takes 24 hours to take effect and you must wait 14 days before you can
-        change the storage billing model again.
+        Set the value to one of ``'LOGICAL'``, ``'PHYSICAL'``, or
+        ``'STORAGE_BILLING_MODEL_UNSPECIFIED'``. This change takes 24 hours to
+        take effect and you must wait 14 days before you can change the storage
+        billing model again.
 
         See `storage billing model
         <https://cloud.google.com/bigquery/docs/reference/rest/v2/datasets#Dataset.FIELDS.storage_billing_model>`_
@@ -788,13 +789,11 @@ def storage_billing_model(self):
     def storage_billing_model(self, value):
         if not isinstance(value, str) and value is not None:
             raise ValueError(
-                "storage_billing_model must be a string (e.g. 'LOGICAL', 'PHYSICAL'), or None. "
-                f"Got {repr(value)}."
+                "storage_billing_model must be a string (e.g. 'LOGICAL',"
+                " 'PHYSICAL', 'STORAGE_BILLING_MODEL_UNSPECIFIED'), or None."
+                f" Got {repr(value)}."
             )
-        if value:
-            self._properties["storageBillingModel"] = value
-        if value is None:
-            self._properties["storageBillingModel"] = "LOGICAL"
+        self._properties["storageBillingModel"] = value
 
     @classmethod
     def from_string(cls, full_dataset_id: str) -> "Dataset":

tests/unit/test_dataset.py

@@ -955,7 +955,7 @@ def test_storage_billing_model_setter(self):
     def test_storage_billing_model_setter_with_none(self):
         dataset = self._make_one(self.DS_REF)
         dataset.storage_billing_model = None
-        self.assertEqual(dataset.storage_billing_model, "LOGICAL")
+        self.assertIsNone(dataset.storage_billing_model)
 
     def test_storage_billing_model_setter_with_invalid_type(self):
         dataset = self._make_one(self.DS_REF)